1. Field of the Invention
This invention relates generally to synthesis of a sinusoidal signal that satisfies some specific frequency and phase conditions based on zero-crossings of a pilot signal, and more particularly to digital sub-carrier signal recovery that is often used in stereo Frequency Modulation (FM) receivers.
2. Description of the Related Art
A stereophonic multiplexed (MPX) signal is composed of a sum signal corresponding to the sum of the left and right audio channels, a pilot signal at 19 kHz, and a 38 kHz double-sideband suppressed-carrier (DSBSC) modulated signal of a difference signal corresponding to right channel subtracted from the left channel. The MPX signals are thereafter frequency modulated and transmitted. The pilot signal indicates to a FM receiver that there is stereophonic information at the second harmonic frequency of the pilot signal (i.e. twice the frequency of the pilot signal). A stereo FM receiver which receives the pilot signal may double the frequency of the pilot signal to demodulate the stereophonic information and obtain the left and right audio channels. The pilot signal may also aid the FM receiver in recovery of the sub-carrier signal by serving as a phase reference. Normally, if the recovered sub-carrier signal and the pilot signal are not synchronized in phase, the FM receiver may be unable to perfectly separate the left and right audio channels, resulting in interference with the channels and degradation of the left and right audio channel signals. Therefore, a need exists for a method and apparatus that generates a sub-carrier signal in phase with a pilot signal and maintains phase synchronization.